The sized portions to be fed to the cavity of the forming mould are generally obtained by separation from a strip of extruded material. The strip can have a circular or approximately circular cross-section, which can be full or annular.
To correctly form the article within the compression forming mould the geometrical shape of the sized portion separated from the strip must be regular and free of surface defects.
The separation of the sized portion from the strip hence represents an operation of essential importance.
It should be noted that the sized portion separated form the strip is ductile and sometimes sticky, this fact enabling the ensuing description to be better understood.
It should also be noted that the devices for separating and withdrawing the sized portion are generally positioned in the known art on a turntable which rotates adjacent to the extrusion nozzle, which is fixed and can be positioned below or above the separation and withdrawal devices.
In both cases the withdrawal device consists of a U-shaped element with its arms slightly diverging, associated with pneumatic devices for retaining the sized portion or for releasing it where required, the element presenting its mouth facing forward with reference to the direction of rotation of the turntable on which it is arranged.
More precisely the mouth is positioned “upstream”, the terms “upstream” and “downstream” relating hereinafter to the direction of rotation of the turntable.
Said device is known as the hand-like withdrawal element or simply hand-like element, and will be known as such hereinafter.
The known art comprises devices for cutting and separating the sized portion associated with the extrusion nozzle, and arranged to collaborate with the hand-like element.
According to the known art, said separation or cutting devices are positioned downstream of the hand-like element, and are rigid therewith. Systems are also known consisting of a sort of spring cutter in which the element which cuts off the sized portion simultaneously propels the plastic material into the mould.
The known solutions suffer from a series of drawbacks deriving from the fact that the action of the hand-like element causes deformation or inaccuracy in the sized portion which negatively influences the quality of the moulded article.
The velocity with which the material strip is cut through depends on the machine productivity and hence cannot be adjusted to the desired optimum velocity for separating the sized portion from the strip.
In the known art, the sized portions separated from the strip first come into contact with the hand-like element, which pushes them tangentially forward relative to the extrusion nozzle, and then come into contact with the cutting devices consisting of a cutting blade which shaves the material on the extrusion nozzle only in that part of the sized portion which when separated is distant from the extrusion nozzle.
That portion of the sized portion cut off may also appear torn because if the cutting velocity is not suitable, elastic viscose movements predominate within the molten material which lead to tearing instead of cutting of the material in the separation area.
That portion of the sized portion which is the last to abandon the extrusion nozzle is therefore never subjected to correct cutting action, either because the cutting devices act when said portion has already been separated from the strip, or because they act at an inconvenient velocity. Said upper portion therefore presents a surface with traces of the tearing action due to the passage of the hand-like element in contact with the extrusion nozzle.